More than 50 lines of mice carrying histocompatibility mutations have been or are being developed in this laboratory. Such mutants have become increasingly useful in a wide range of immunogenetic and immunobiological research. We propose to establish and maintain the current lines, to add to this unique collection as new mutations are recovered, to identify and map the mutated genes insofar as is practical, to utilize the mutants in immunogenetic studies (our own and those of collaborators), and to ultimately make the completed and defined mutant lines available to other investigators. We currently hold 20 H-2 and about 30 non-H-2 mutations. We plan to map the H-2 and non-H-2 mutations, characterize their effects on skin graft rejection and on resistance to certain transplantable tumors. We also plan to study mutant histocompatibility genes whose expression is multigenically determined and can be altered by changes in the genetic background of the graft donor. In addition to our own studies, we will systematically supply mutants to certain collaborators for their studies on the mutants with respect to serological changes, in vitro cell mediated reactivity, structural analysis of the mutant gene products, and tumor immunology.